Magnet coil



Oct. 1l, 1949. n. E. FERRls Erm.

MAGNET COIL` Filed July 20, 1945 Patented Oct. 11, 1949 UNITED STATES PATENT OFFICE yilniomrr con.

Application July 20, 1945, Serial No. 606,064

2 Claims. (Cl. 175-21) Our invention relates, generally, to magnet coils and, more particularly, to coils for electromagnetic relays and switches.

Sometimes a magnet coil which is designed to operate on a relatively low voltage is subjected to a relatively high voltage because of trouble in some other part of the control system in which the coil is utilized. The high voltage causes the coil to become so hot that itV explodes or bursts into fiame, thereby setting re to surrounding apparatus. An external fuse in the coil circuit is not satisfactory for some applications because of the extra connections required for the fuse and the blowing of the fuse by temporary overcurrent conditions which are not dangerous to the coil itself.

An object of our invention is to prevent a magnet coil from becoming overheated when subjected to excessive voltage.

Another object of our invention is to protect a magnet coil against excessive voltage without requiring an external fuse in the coil circuit.

A further object of our invention is to provide a magnet coil in which a fusible portion constitutes part of the coil winding.

Other objects of our invention will be explained fully hereinafter or will be apparent to those skilled in the art.

In accordance with one embodiment of our invention, a relatively short length of wire of a smaller size than that utilized for the coil winding is encased in a reproof sleeve and Wound in as a part of a magnet coil. The reduced section of wire fuses open when the coil is subjected to a relatively high voltage, thereby protecting the remainder 01' the coil winding and preventing setting nre to surrounding apparatus.

For a fuller understanding of the nature and objects of our invention, reference may be had to the following detailed description, taken in conjunction with the accompanying drawing, in which:

Figure 1 is a view, in plan, of a magnet coil embodying our invention;

Fig. 2 is an enlarged view, in section, taken along the line II--II of Fig. 1;

Fig. 3 is an enlarged view of a portion of the coil winding, and

Fig. 4 is an enlarged detail view, showing the Joint b'etween the two wires o1' a different size.

Referring to the drawing, a magnet coil I shown therein comprises a plurality of turns of insulated wire II of a predetermined diameter and substantially one turn of wire I2 of a smaller diameter than the wire il. One end of the smaller wire I 2 is connected to one end of the larger wire II and constitutes part of the coil winding.

As shown, the coil may be made by winding the wire II on a tube Il in the usual manner. A layer oi' fireproof insulation I4, such as sheet mica, is then wound around the wire I I; the mica being of suillcient length to overlap the joint I5 between the smaller wire I2 and the larger wire I I.

As shown in Fig. 4, the Joint I5 between the wires II and I2 may be formed by lapping the end of the wire I2 back on itself to form two strands, twisting the two wires together and then soldering the end of the wire II to part of the doubled-back portion of the wire I2. The smaller wire I2 may be encased by a sleeve I6 of flexible flreproof insulation, such as fiberglass, which extends over the joint I5 between the wires II and I2. As shown in Fig. 1, exposed ends of the wires I I and I2 are available for connecting terminal leads in the usual manner.

As explained hereinbefore, the short length of wire I2 fuses open when an excessive voltage is applied to the coil, thereby protecting the remainder of the coil and preventing res or explosions. If desired, the outside insulation may be removed from the coil and the length of small wire replaced, thereby making the coil again available for service at a relatively small expense.

Magnet coils of the present type may `be utilized in electromagnetic relays and switches of various types. They are particularly suitable for use on the control apparatus utilized in street cars and trolley buses where the apparatus is mounted in a relatively small amount of space,

and there is danger of a flre being started by the failure of one coil when it is subjected to an excessive voltage. The present structure has been found to be more satisfactory than attempting to protect the apparatus by means of fuses, which will blow under temporary overload conditions not dangerous to the magnet coils. The blowing of fuses results in the stopping of the operation of the vehicle, thereby causing considerable inconvenience. f

Since numerous changes `may be made in the above described construction, and different embodiments of the invention may be made without departing from the spirit and scope thereof, it is intended that all matter contained in the foregoing description or shown in the accompanying drawing shall be interpreted as illustrative and not in a limiting sense.

We claim as our invention:

1. A magnet coil comprising a plurality of turns of wire of a predetermined diameter, a length oi fusible wire made of the same material and being of a smaller diameter than the rst-named wire, said smaller wire being disposed on the outside of the larger wire and connected to the larger wire to constitute part of the coil winding, one end of the smaller wire being lapped back on itself and twisted to one end of the larger wire to form a joint therebetween, and a layer of flreproof insulation disposed between the smaller and the larger lo wires and overlapping the joint between said wires.

2. A magnet coil comprising a plurality of turns of wire of a predetermined diameter, a length of fusible wire made of the same material and be ing of a smaller diameter than the first-named wire, said smaller Wire being disposed on the outside of the larger wire and connected to the larger wire to constitute part of the coil winding, one end of the smaller wire being lapped back on itself and twisted to one end of the larger Wire to form a joint therebetween, a layer of reproof insulation disposed between the smaller and the 4 larger wires and overlapping the Joint between said wires, and a sleeve of ilreproof insulation encasing said smaller wire.

RALPH E. FERRIS. .JOHN R. SHIRLEY.

REFERENCES CITED The following references are o! record in the file of this patent:

UNITED STATES PATENTS Number Name Date 668,030 Wood Feb. 12, 1901 714,226 Northall Nov. 25, 1902 928,136 Lacy July 13, 1909 1,455,781 Dubilier May 22, 1923 1,722,362 Wiley July 30, 1929 1,872,369 Van Sickle Aug. 16, 1932 2,377,420 Hill June 5, 1945 FOREIGN PATENTS Number Country Date 454.178 Great Britain Sept. 25. 1936 

